The present invention relates to a laser diode driver circuit for driving a laser diode and specifically to a laser diode driver circuit suitable for an optical communication device which performs data communication by driving a laser diode to emit light according to an input signal.
FIG. 10 shows a circuit structure of a conventional laser diode driver circuit. The conventional laser diode driver circuit 10 includes: a current source 11 for generating a laser drive current for driving a laser diode 101 which performs current-to-light conversion; a switch transistor 12a which has a drain connected to the laser diode 101 and a source connected to the current source 11; a switch transistor 12b which has a drain connected to power supply VDD and a source connected to the current source 11; an APC (Auto Power Control) circuit 13 for feeding back a monitoring result of laser light 20 which is obtained by a photodiode 102 to the current source 11 to adjust the magnitude of the laser drive current such that the amount of the laser light 20 is always kept constant.
Differential switching signals IN0+ and IN0− are input to the gates of the switch transistors 12a and 12b, respectively. When the switch transistor 12a is ON and the switch transistor 12b is OFF, an electric current passes through the laser diode 101 so that the laser diode 101 emits the laser light 20. When the switch transistor 12a is OFF and the switch transistor 12b is ON, no electric current passes through the laser diode 101 so that the laser diode 101 does not emit the laser light 20.
In actuality, the operation of the laser diode driver circuit 10 having the above structure is influenced by the inductance and capacitive component parasitic to packages, leads and bonding wires of the laser diode 101 and the laser diode driver circuit 10 itself and the capacitive component parasitic to the output of the laser diode driver circuit 10. This influence is negligible when the operation frequency is relatively low. However, when the operation frequency is relatively high, especially when the operation frequency is higher than 1 Gbps, ringing which occurs at rising and falling edges of the output waveform of the laser diode driver circuit 10 cannot be neglected. For example, in the case where the laser diode driver circuit 10 is used in an optical communication device, if ringing occurs in emission of the laser diode 101, the reliability of high speed data communication is deteriorated. In the worst case, a critical defect, such as a data dropout, error data, or the like, occurs.
In view of such, inserting a filter circuit, which is formed by a capacitive element 151 and a resistive element 152 connected in series, between the drain of the switch transistor 12a and the drain of the switch transistor 12b to suppress the influence of ringing has conventionally been proposed (see, for example, Japanese Unexamined Patent Publication No. 2000-203080 (page 5, FIG. 1)).
In the above ringing suppression method, the time constant of the filter circuit is fixed to a constant value. Thus, the output waveform becomes blunt in return for suppression of the influence of ringing. Conversely, ringing or overshoot and undershoot remain unremoved in return for optimization of the rising and falling of the output waveform.
FIG. 11 is a graph showing the input/output characteristic of the conventional laser diode driver circuit 10. In FIG. 11, line segment (a) shows an input signal to the conventional laser diode driver circuit 10; line segment (b) shows the output waveform obtained when the laser drive current is small; and line segment (c) shows the output waveform obtained when the laser drive current is large. As seen from line segment (b), when the laser drive current generated by the current source 11 is relatively small, overshoot and undershoot readily occur. As seen from line segment (c), when the laser drive current generated by the current source 11 is relatively large, the output waveform becomes blunt. Thus, when the laser drive current varies over a relatively wide range, it is difficult to obtain an optimum output waveform. In the case where a laser diode having a deteriorated output waveform is used in optical data communication, high speed response is not achieved, and data dropout, error data, or the like, occurs. As a result, the reliability of communication data decreases.